1. Field of the Invention
The invention relates generally to modulation systems and more particularly to an IQ modulation system using feedback for calibrating an IQ modulated output signal while the system is on-line.
2. Description of the Prior Art
In-phase (I) and quadrature phase (Q) modulators are commonly used for generating digital modulation such quadrature phase shift key (QPSK). QPSK is easily visualized in the IQ plane as an IQ diagram that is a square centered at the zero signal point and having modulation states at each of the corners. Many other common modulations, such as sixteen quadrature amplitude modulation (16 QAM), 64 QAM, and 256 QAM, are possible using basically the same IQ modulator by controlling the amplitude of I and Q data streams driving the modulator. Unbalanced formats, where the I modulation amplitude and the Q modulation are not equal, are also possible using the same IQ modulator.
Such IQ modulators are subject to several well-known errors. One error is due to carrier signal that leaks through the IQ modulator into the modulated output signal. The carrier leakage offsets the IQ diagram of the signal away from the zero signal point and is therefore sometimes termed an offset error. In general, the offset error has both an I offset error in the I dimension of the IQ plane and a Q offset error in the Q dimension of the IQ plane. Another error, termed quadrature error or I/Q phase error, occurs because the I modulation and the Q modulation from the IQ modulator are not exactly in quadrature. Another error, termed I/Q gain imbalance, occurs because the I modulation component and the Q modulation component from the IQ modulator do not have a desired ratio. For standard QPSK, 16 QAM, 64 QAM, and 256 QAM the desired ratio is one. However, unbalanced modulation formats having ratios other than one are possible.
Several approaches have been used for correcting the errors in IQ modulators. One approach is to observe the output of the IQ modulator on a vector network analyzer for certain test inputs while either adjusting parameters of the IQ modulator or adjusting the circuits driving the IQ modulator. Then, when the adjustments yield a satisfactory result, they are fixed in place and the IQ modulator is put into service. This approach has several disadvantages. Expensive test equipment is required. The parameters of the IQ modulator can drift causing the performance of the IQ modulator to degrade after the adjustments are fixed. And, the IQ modulator must be out of service while the calibration is performed. Another approach disclosed by Edwards et al. in U.S. Pat. No. 4,717,894 uses a scalar detector in place of the vector network analyzer. This approach eliminates the need for expensive test equipment. However, the Edwards approach also requires that the IQ modulator be out of service while it is being calibrated. It should be appreciated that a communication system cannot easily be taken off-line for calibration and adjustment.
There is a need for a modulation system that can be calibrated without taking the modulation system off-line.
It is therefore an object of the present invention to provide an apparatus and method using a scalar detector and feedback for on-line calibration of a modulation system.
Briefly, in a preferred embodiment, a modulation system of the present invention includes digital filters for converting in-phase (I) and quadrature phase (Q) data bit streams into filtered multilevel I and Q digital data streams, digital-to-analog converters for converting the data streams from digital to analog form, and an IQ modulator for converting the analog I and Q data streams into a modulated output signal that can be represented with an IQ diagram.
In order to reduce errors in the modulated output signal without talking the system off-line, the system also includes an on-line correction data state detector, an IQ correction code, and a scalar amplitude detector. Particular modulation states and transition locations of the IQ diagram are selected for consideration. The digital filters include forward shifting memories having several samples of the data bit streams for each data bit time. The data state detector monitors the samples in the memories, termed data states, and detects the presence of particular data states that are expected to provide the particular modulation states or transition locations. The amplitude detector monitors the modulated output signal and provides representative detected magnitudes. When one of the particular data states is detected the IQ correction code is triggered to receive the detected magnitude. The IQ correction code compares the detected magnitude to magnitudes that have been detected and stored previously and generates calibration adjustments from the comparisons for correcting errors in the modulated output signal.
The primary errors requiring correction are carrier leakage, termed I and Q offsets; deviation from quadrature between I and Q modulation, termed I/Q phase error; and amplitude imbalance between I and Q modulation, termed I/Q gain error. Several alternative embodiments can be used for applying the corrective adjustments. In preferred embodiments, I and Q offsets are corrected with I and Q offset adjustment summers in the paths of the I and Q digital or analog data streams for adjusting the balance of I and Q mixers in the IQ modulator. The I/Q phase error is corrected with an I/Q phase adjustment multiplier and summer for the I and Q digital date streams or with a phase shifters in the IQ modulator. The I/Q gain error is corrected with a I/Q gain adjustment multiplier in one or more of the data streams or with a multiplying digital-to-analog converter in one or both of the data streams.
An advantage of a modulation system the present invention is that the modulation system uses an inexpensive scalar detector for calibrating the modulated output signal without interrupting service.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.